1. Field of the Invention
This invention relates to a method for producing rubber articles by cross-linking a caoutchouc mixture based upon powdered caoutchouc.
2. Description of the Prior Art
In the manufacture of rubber articles from the usual baled caoutchouc (bale-caoutchouc), first the so-called "batches" are produced in a kneader. Thereafter, the auxiliary vulcanization agents are mixed-in on a roller, before the actual shaping is done, for example, in an extruded or calender.
The use of powdered caoutchouc for the production of rubber articles was based upon an attempt to save expensive mixture-components and, at the same time, to reduce the time required to produce the raw mixture.
In order to save on the required mixing apparatus, e.g. the kneader and the roller, the so-called "powdered caoutchouc" was developed. In the production of rubber articles from this powdered-caoutchouc, the raw material mixture is melted in a mixer-extruder, mixed and thereafter processed in a compactor to obtain the required density of at least 1.0 g/cm.sup.3. However, the mixer-extruder as well as the compactor can process the powdered caoutchouc mixture only relatively slowly, because the friction forces generated at a rapid process speed cause a strong temperature rise, and thereby the danger exists, that the powdered caoutchouc mixture containing the vulcanization accelerators "jumps" into the reaction state.
It was found in practice, however, that the use of powdered caoutchouc did not produce the expected results, mainly because it was necessary to use the total amount of raw materials, called for in the mixture-formula, right from the start of the production of the mixture. However, in the case of sulphur cross-linking, a mixture of this kind contains accelerators or, in the case of C-C cross-linking, organic peroxides, these materials can be subjected only to slight increases in temperature, since as soon as a specific temperature limit is exceeded, the mixture becomes activated. This leads to premature cross-linking or vulcanization, after which rubber articles can no longer be produced.
This method also did not prove to be successful in practice, due to the fact that two mixing machines were required, i.e. the mixer-extruder and the compactor, and furthermore, that the process speed of the powdered caoutchouc mixture must be kept very low, because of the explained danger to "jump" into the reaction state.
Another disadvantage of this known method for producing rubber articles from powdered caoutchouc is that it is impossible to process a quantity of loose mix-ingredients directly into a semi-finished product, for example by extrusion or calendering; instead the mixture must first pass through a so-called "compactor" which reduces the density of the mixture from about 0.5 g/cm.sup.3 to the necessary maximum of at least 1.0 g/cm.sup.3. This additional operation not only complicates the known method, but also increases the total processing time.